This invention relates to a rare earth silicate phosphor co-activated by cerium and terbium, which, when excited by ultraviolet rays, efficiently emits green rays.
Some phosphors activated by a rare earth element such as terbium are known to efficiently to emit green rays when excited by ultraviolet rays having a particular wavelength range. The green rays thus emitted are found to have a line spectrum pattern. The U.S. Pat. No. 3,523,091 sets forth lanthanum silicate, yttrium silicate and lanthanum-yttrium silicate all activated by terbium. British Patent No. 1,452,083 describes that a terbium-activated aluminate phosphor, for example, (Ce.Tb)MgAl.sub.11 O.sub.19 efficiently emits green rays when excited by ultraviolet rays. An excitation spectrum of the prior art phosphor has a peak in the proximity of a wavelength of 254 nanometers (hereinafter abbreviated as "nm") or 300 nm. The peak extends over a relatively narrow region, and excitation efficiency particularly at a wavelength of 365 nm is noticeably low.
Minoru Watanabe, one of the present inventors, previously proposed in the Japanese patent application disclosure No. 83,773/1973 a flying spot scanner tube whose phosphor layer is composed of yttrium silicate co-activated by cerium and terbium. The phosphor used with the proposed scaner tube is of a different type in that the previously proposed phosphor emits a light when excited by electron beams.
The disclosed literature only sets forth a flying spot scanner tube using a phosphor which emits a light when excited by electron beams, and makes no reference to a phosphor which, when excited by ultraviolet rays, emits green rays and is particularly adapted for use with a fluorescent lamp.
It is accordingly an object of this invention to provide a phosphor capable of efficiently emitting substantially green visible rays when excited by ultraviolet rays.
Another object of the invention is to provide a rare earth silicate phosphor whose excitation spectrum has a peak at both wavelength of 253.7 nm and 365 nm.
Still another object of the invention is to provide various types of fluorescent lamp which display high efficiency and high color rendering due to application of the rare earth silicate phosphor.